Delivering a Modifying Element to Metal Nanoparticles via Support: Pt-Ga Alloying during the Reduction of Pt/Mg(Al,Ga)O_x Catalysts and Its Effects on Propane Dehydrogenation

摘要

The alloying of Pt with Ga delivered from a hydrotalcite-like support was investigated as a strategy to produce bimetallic catalysts for propane dehydrogenation. A series of Pt/Mg(Al,Ga)O_x catalysts (2-3 wt % Pt, Ga/Pt molar ratios between 0 and 10) and a model Pt/Ga2O3 catalyst (4 wt % Pt, Ga/Pt molar ratio of 50) were characterized by means of X-ray diffraction (XRD), transmission electron microscopy, and activity measurements (873 K, W_(cat) /F_(C3H8,0) =25kg_(cat) ·s·mol ~(-1) and P_(C3H8,0) = 5 kPa at a total pressure of 101.3 kPa). XRD patterns taken during temperature-programmed reduction in 5% H2/He and isothermal reduction/oxidation cycling between 5% H2/He and 20% O2/N2 at 873 K revealed dynamic alloy formation and segregation that depended upon the gas environment and Ga content. Alloying on the Pt/Mg(Al,Ga)O_x catalyst with a Ga/ Pt ratio of 2 could not be observed by XRD. For a Ga/Pt ratio of 10, an alloy with a diffraction peak at 40.2° was formed during the initial reduction. After subsequent reduction/oxidation treatments, this catalyst evolved toward a stable periodic cycling between pure Pt and one or more Pt-Ga alloys with characteristic peaks at 40.2° and 46.5°. The exact composition of the Pt-Ga alloy(s) could not be identi?ed. On the model Pt/Ga2O3 catalyst, an alloy was formed with the same characteristic peak at 40.2° as on the Ga-rich Pt/Mg(Al,Ga)O_x.In addition, another Pt-Ga alloy appeared on the Pt/Ga2O3 catalyst, which was identi?ed as a stoichiometric PtGa phase. These alloys were formed on Pt/Ga2O3 at a lower temperature than on Pt/Mg(Al,Ga)O_x and they were stable during the reduction/ oxidation cycling. Catalytic activity measurements demonstrated that the formation of Pt-Ga alloys on the Pt/Mg(Al,Ga)O_x sample with a Ga/Pt ratio of 10 and on the Pt/Ga2O3 catalyst led to pronounced enhancement of the initial selectivity toward propylene, but lower activity per exposed Pt atom.